1) Field of the Invention
The present invention relates to an optical scanning system for deflecting a laser beam stepwise, to scan a fine zone with the stepwise deflected laser beam.
2) Description of the Related Art
Developments in precision manufacturing now demand a very high accuracy in the working of various precision components. For example, in the manufacturing of a precision component such as an integrated circuit, a magnetic head or the like, the working accuracy must be on the order of less than 1 .mu.m. In particular, a fine circuit pattern of the integrated circuit and a fine gap of the magnetic head must be worked on the order of less than 1 .mu.m. Accordingly, in the manufacturing of the precision component, it is necessary to frequently measure the dimensions of the fine pattern to control the quality of the precision component.
British Patent No. 2,147,097 and U.S. patent application Ser. Nos. 014,619 and 151,699 disclose an optical measurement apparatus for dimensionally measuring a fine object or pattern formed on or in the precision component, which comprises a laser beam source for emitting a laser beam having a Gaussian distribution, and an optical scanning system for scanning a measuring zone, including the fine pattern, with the laser beam. The optical scanning system includes a light deflection means such as an acoustic-optical device for deflecting the laser beam stepwise in a fixed plane perpendicular to the measuring zone including the fine pattern. Namely, the measuring zone is scanned with the stepwise deflected laser beam along a scanning line included in the fixed plane. Note that the acoustic-optical device is able to stepwise deflect the laser beam so that the laser beam spot is gradually shifted along the measuring zone by steps of about 0.01 .mu.m.
The optical measurement apparatus further comprises a detector for detecting a light intensity of a laser beam reflected from the measuring zone at each of the scanning steps, and a processor for preparing a reflected light intensity distribution on the basis of data obtained from the detector during the scanning operation, and for processing the reflected light intensity distribution to determine a dimension of the fine pattern to be measured along the scanning line. This measurement is based upon the phenomenon that the laser beam is reflected in a different manner at the fine object or pattern than at other areas except therefor. For example, when the fine pattern has a reflectivity different from that of the other areas, a light intensity of the laser beam reflected at the fine pattern is different from that of the laser beam reflected at the other areas. Accordingly, the reflected light intensity distribution as mentioned above represents a dimension of the fine pattern to be measured along the scanning line, whereby the fine pattern dimension can be determined by suitably processing the reflected light intensity distribution, as disclosed in British Patent No. 2,147,097 and U.S. patent application Ser. Nos. 014,619 and 151,699.
In the conventional optical scanning system, the laser beam is deflected only on the fixed plane, as mentioned above. Namely, the laser beam scan is carried out only along a line which is fixed with respect to the optical scanning system. Accordingly, the fine pattern to be measured must be shifted with respect to the optical scanning system before several dimensions of the fine pattern can be measured, and therefore the optical measurement apparatus is provided with a mechanically movable table on which the precision component is mounted as a sample. The movable table can be two-dimensionally shifted with respect to the optical scanning system, so that the presision component mounted thereon is brought to a measuring position at which the fine pattern of the precision component is scanned with the laser beam.
Nevertheless, since a mechanical means is used to move the table, it is difficult to precisely and quickly position the table as desired, because the operation of the mechanical means is inevitably accompanied by vibration.
Accordingly, a two-dimensional optical scanning system has been developed for measuring several dimensions of the fine pattern without moving the pattern. This two-dimensional scanning system includes a second light deflection means such as a galvano mirror for a stepwise deflection of the laser beam in a direction perpendicular to the scanning direction, in which the laser beam is deflected by the first light deflection means or an acoustic-optical device. Namely, in the two-dimensional optical scanning system, a raster scan can be carried out by the second light deflection means or the galvano mirror, whereby several dimensions of the fine pattern can be quickly measured without moving the fine pattern.
Nevertheless, although there is a demand for a quick measurment of dimensions of the fine pattern in two directions perpendicular to each other, this demand cannot be satisfied by the two-dimensional optical scanning system as mentioned above because the galvano mirror cannot gradually deflect the laser beam by the same fine distance as can the acoustic-optical device. Namely, the fine pattern must be turned through a 90-degree arc after a dimension of the fine pattern has been measured in one of the two directions perpendicular to each other.
As apparent from the above, the precision component must be moved by the table to the measuring position at which the fine pattern of the precision component is scanned with the laser beam, before the measurement can be performed in the optical measurement apparatus. But this positioning of the precision component is very difficult because of the extremely small size of the fine pattern.